1. Field of the Invention
This invention pertains to the application of cryogenic technology to life support systems and, more particularly, in one of its aspects to an insulated pressure vessel, such as a dewar, for storing and delivering a cryogenic fluid. In another of its aspects, it refers to an improved process for rapidly filling the dewar with a cryogenic fluid.
2. Description of the Prior Art
"Cryogenic" is a term used to describe physical conditions at temperatures less than approximately 123K (-151.degree. C., -239.degree. F.). A "cryogenic fluid" may be defined as a fluid whose temperature is less than approximately 123K (-151.degree. C., -239.degree. F.) that boils at temperatures less than approximately 110K (-262.degree. F., -163.degree. C.) at atmospheric pressure, and a cryogenic fluid may therefore be either a gas or a liquid. Although these definitions are adequate for many applications, the terms are capable of many definitions and the use of the terms herein should be construed consistently with the many definitions accepted by those in the art. Examples of cryogenic fluids include both nitrogen and oxygen, the primary components of "liquid air". The term "cryogen" as used herein shall refer to a cryogenic fluid and the term "cryogenic technology" shall refer to knowledge, techniques, and equipment for harnessing physical properties of cryogenic fluids to practical applications.
A wide variety of diverse fields employ cryogenic technology, and portable life support systems are experiencing a resurgent interest in cryogenic technology. Many portable life support systems store liquid cryogen in a vacuum insulated pressure vessel called a dewar from which liquid cryogen is delivered to other parts of the life support system. One such system is disclosed and claimed in U.S. Pat. No. 5,361,591 issued Nov. 8, 1994 to Oceaneering International, Inc. as assignee of the inventor Bruce D. Caldwell. An inner pressure vessel is typically jacketed by an insulative housing, the space between the pressure vessel and the insulative housing being evacuated and sometimes filled with multi-layered insulation or reflective powders.
Any dewar in a portable life support system employing cryogenic fluids will contain gas and, if filled, liquid cryogen. With the exception of portable life support systems used in micro-gravity or zero-gravity environments, most portable life support system dewars rely on gravity to separate liquid cryogen from gaseous cryogen. Gravity separation is advantageous because the cryogenic fluid can be pressurized to provide a motive force in delivering the liquid cryogen from the dewar. One may therefore take advantage of the natural properties of the cryogen to deliver the liquid from the dewar by pressurizing the separated gas within the dewar's pressure vessel.
Some current efforts at portable life support system design such as that disclosed in the '591 Caldwell patent focus on using liquid cryogen as part of a cooling loop regulating the user's body temperature. Heat exchange in such a cooling loop cools the user through an intermediate cooling loop while warming the liquid cryogen, generally converting the liquid cryogen to gas. If the liquid cryogen is "liquid air", a breathing air supply is provided for the system's user by the vaporization of the cryogen.
This type of portable life support system requires an uninterrupted flow of liquid cryogen from the dewar and the ability to gauge the dewar's cryogen content. Additionally, liquid withdrawal is necessary to (1) control the pressure of the dewar contents, (2) utilize the phase change from liquid to gas that provides one-half the cooling capacity of the system, and (3) deliver a consistent component mixture for the air in the breathing air supply. Thus, gravity separation can be very advantageous.
The drawback to gravity separation is that the liquid cryogen's position shifts within the dewar when the orientation of the dewar is changed with respect to gravity. The dewar for a portable life support system is usually worn on the back of the system user and, whenever the user bends at the waist, the orientation of the dewar with respect to gravity changes. Such changes in orientation can also occur by body movements other than those at the waist. These changes can occur in one, or both, planes of movement: (1) forward and back, and (2) side to side.
The shift in position by the dewar's liquid contents can expose a typically fixed intake port through which liquid cryogen is delivered in a standard upright dewar to the gaseous cryogen in the dewar. When the port is exposed, the pressurized gaseous cryogen escapes through the port. This depressurizes the dewar, eliminates the motive force and thereby interrupts the delivery of liquid cryogen. For instance, if someone wearing a portable life support system stoops or bends over as if to lift something, the intake port may become exposed and allow the pressurized gas to escape and interrupt the liquid cryogen's delivery until the port is once again immersed in the liquid cryogen and pressure is restored to the dewar.
The positional shift of the dewar's contents also causes problems in ascertaining the volume of liquid cryogen remaining in a standard upright dewar. Because it is desirable to fully utilize the contents of the dewar and, more importantly, to provide the user at all times with an accurate determination of the remaining cryogen in the dewar, i.e., the dewar capacity, an accurate determination of liquid cryogen levels in the dewar must be available at virtually all times. Furthermore, reliability in the cryogen capacity is entirely dependent on the dewar's ability to provide for complete withdrawal of the cryogen stored therein. Complete utilization of the dewar contents further permits for a reduction in the overall weight of the dewar because there is no need for a safety margin in the cryogen to accommodate inaccurate gauging. The reduction in weight further results on increased capacity because the user's metabolic rate, and thus his/her cryogen consumption rate, is reduced under the lighter load. Thus, for example, a fireman entering a burning building obviously needs to know how much liquid cryogen is available in the dewar for cooling and/or breathing purposes, since the liquid quantity can be equated to a capacity or duration time for the user to monitor.
Current techniques employ a capacitance gauge fixed in a gravity vector in the dewar which distinguishes gas from liquid by their differing dielectric constants. The capacitance of the gauge varies with the level of liquid, so the shifting of liquid cryogen within the dewar caused by user movement also prohibits accurate determination of liquid cryogen levels in the dewar.
It is an object of this invention to provide an apparatus for the delivery of a liquid cryogenic fluid without interruption resulting from changes in orientation of the apparatus with regard to gravity or other external acceleration forces.
It is a further object of this invention to provide means for completely withdrawing the liquid cryogen from such an apparatus, especially during changes in orientation relative to gravity.
It is a further object to provide a capacitance gauge that accurately measures the quantity of liquid cryogen in the vessel regardless of the vessel's orientation.
It is a further object to provide an apparatus containing an insulated vessel for storing the liquid cryogen that is structurally independent.
It is a further object to provide such a vessel having a shape that maximizes its structural integrity while preserving its useful volume for storing liquid cryogen.
It is a further object to provide such an insulated vessel that requires no substantial supplemental structural support members, and as a result is easier to manufacture and exhibits a reduced weight, whereby the weight of the overall apparatus is reduced and the physical stress imposed on the user is also reduced.
It is a further object to provide such an apparatus having a minimum of sealed surfaces, particularly sealed rotary joints, so that leakage of the liquid cryogen is minimized and a liquid intake means in the apparatus can move freely under reduced friction to follow the liquid cryogen as the liquid moves within the vessel under an external force such as gravity.
It is a further object that the intake means have a minimum of relatively moving components to further reduce friction so that the intake means can freely follow the liquid cryogen and the weight of the apparatus is reduced.
It is a further object that the apparatus include an offset means, referred to as a "kicker," in the vessel for ensuring that the intake means quickly reacts to follow the liquid cryogen within the vessel as the liquid is moved by orientation of the apparatus relative to the gravity vector.
It is a further object that the insulated vessel be mounted within the apparatus in a manner that minimizes the amount of heat transferred through the apparatus to the vessel.
It is a further object to provide means for rapidly filling the vessel with liquid cryogen in a highly efficient manner.